1. Field of the Invention
The present invention relates to a process for producing a gradient index optical element and a gradient index optical element produced thereby which is suitable for use in optical devices such as a camera, a microscope, an endoscope and an electronic imaging optical system. More particularly, the present invention is concerned with a process for producing a gradient index optical element in which a porous body containing a silicon component is immersed in a solution containing hydrofluoric acid and thereafter in a solution containing a metal alkoxide or derivative thereof and concerned with a gradient index optical element produced thereby.
2. Discussion of Related Art
The gradient index optical element is one whose medium itself has been provided with refractive power by causing the medium to have a refractive index distribution. The gradient index optical element has excellent aberration suppressing capability to thereby enable reducing the number of constituent lenses, so that it is attracting attention as an optical element which is indispensable in next-generation optical systems.
The known gradient index optical elements can be classified as follows, depending on the Abbe's number change and the refractive index change in the radial direction thereof. That is, the gradient index optical element whose refractive index decreases from a glass center to a periphery thereof is designated the gradient index optical element of the positive refractive power type, and the gradient index optical element whose refractive index contrarily increases is designated the gradient index optical element of the negative refractive power type. With respect to the chromatic dispersion accompanying the refractive index change, the chromatic dispersion distribution in which the Abbe's number is decreased with the increase of the refractive index is designated the positive dispersion distribution, the chromatic dispersion distribution in which the Abbe's number is contrarily increased with the increase of the refractive index is designated the negative dispersion distribution and the chromatic dispersion distribution in which the Abbe's number has no substantial change irrespective of the increase of the refractive index is designated the low dispersion distribution. Thus, combinations of the refractive index change and the Abbe's number change give, for example, an optical element of a negative dispersion distribution of the positive refractive power type. The chromatic dispersion distribution in which the refractive index is scarcely changed and the Abbe's number is greatly changed is designated the super dispersion distribution.
In the field of developments of such optical elements, for example, Microoptics News, vol. 9-3, pp. 13-18 (1991), Preprints of papers 127-128 Presented before 1992 Japan Optics Symposium, U.S. Pat. No. 5,166,827 and SPIE, vol. 1780, pp. 456-463 (1992) disclosed that the low or negative dispersion distribution is advantageous in the gradient index optical element of excellent chromatic aberration suppressing capability which is applicable to white light sources. Further, Applied Optics, vol. 25 no. 18, pp. 3351-3355 (1986) described that the gradient index optical element of the negative refractive power type has excellent chromatic aberration suppressing capability. Still further, Japanese Patent Application Laid-Open Specification No. 148405/1994 disclosed the composition realizing the super dispersion distribution. In the production of the above gradient index optical element having excellent chromatic aberration suppressing capability, it is requisite that the concentration of a metal component such as Ti, Nb, Ta or Zr be continuously increased from a glass center toward a periphery thereof, namely, have a concave distribution.
These gradient index optical elements are produced by, for example, the customary sol gel process, ion exchange process or molecular stuffing process. Among these processes, the sol gel process is attracting intense attention because a glass of high aperture ratio can be obtained and a polyvalent metal oxide can be provided with a distribution to thereby enable giving variation to the optical properties of the obtained gradient index optical element.
Japanese Patent Application Laid-Open Specification No. 108626/1992 disclosed a process for producing the gradient index optical element of the negative refractive power type in which a porous body is immersed in a distribution imparting solution and cooled to thereby stop the move of the distribution imparting solution according to the sol gel process. On the other hand, Japanese Patent Application Laid-Open Specification No. 306126/1993 disclosed a process for producing the gradient index optical element of the negative refractive power type in which a metal salt is incorporated in a porous body from its periphery.
Japanese Patent Application Laid-Open Specification No. 55339/1992 disclosed a process comprising immersing a gel containing a first dopant in a suitable leaching solution to thereby provide the first dopant with a distribution, followed by providing a second component with a distribution. Japanese Patent Application Laid-Open Specification No. 292624/1987 described a process for producing SiO.sub.2 --TiO.sub.2 glass in which a porous body is immersed in a Ti-containing solution to thereby cause the porous body to uniformly contain the Ti component. Japanese Patent Application Laid-Open Specification No. 83719/1992 described a process for producing the gradient index optical element of the positive refractive power type in which a wet porous body of SiO.sub.2 --TiO.sub.2 is immersed in a fluorine-containing solution to thereby leach out the Ti component, so that the optical element has a refractive index distribution having a convex profile in the radial direction thereof.
The application of the process of Japanese Patent Application Laid-Open Specification No. 306126/1993 to the production of the gradient index optical element in which a metal component such as Ti, Nb, Ta or Zr having intense influence on the refractive index is provided with a concave distribution in the radial direction of the porous body is difficult because a suitable salt is not available. The terminology "suitable salt" used herein means the salt which can be easily handled, can be dissolved in an available solvent, can easily control chemical reaction and can produce a homogeneous sol when being used as a material of the sol gel process.
The process of Japanese Patent Application Laid-Open Specification No. 108626/1992 has drawbacks in that metal components are precipitated because of substantial changes of the solubilities thereof which occur in accordance with temperature changes, selection of an employed solvent is difficult and further the reproducibility of the resultant refractive index distribution is not good. In the process of Japanese Patent Application Laid-Open Specification No. 55339/1992, the realization of a concave metal concentration distribution is difficult although the first dopant can be easily provided with a convex metal concentration distribution. On the other hand, the use of the process of Japanese Patent Application Laid-Open Specification No. 292624/1987 for reducing the time of immersion in the titanium-containing solution to there-by provide titanium with a concave distribution in the radial direction encounters problems such that not only can only a small proportion of titanium component contribute to the network because of strong Si--O--Si bonds but also part of the titanium component which does not associate with the Si--O--Si bonds aggregates together at the time of sintering of the porous body to thereby cause precipitation of anatase crystals with the result that the obtained optical element is devitrified. The process of Japanese Patent Application Laid-Open Specification No. 83719/1992 has a drawback in that a concave metal concentration distribution cannot be realized although the use of hydrofluoric acid for leaching out the titanium component from the silica titanium gel can naturally provide the titanium with a convex metal concentration distribution.